WASHINGTON - Helicopters descended out of darkness on the most important counterterrorism mission in US history. It was an operation so secret, only a select few US officials knew what was about to happen.

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The location was a fortified compound in the Pakistani town of Abbottabad. The target was Osama bin Laden.

Intelligence officials discovered the compound in August while monitoring an al-Qaida courier. The CIA had been hunting that courier for years, ever since detainees told interrogators that the courier was so trusted by bin Laden that he might very well be living with the al-Qaida leader.

Nestled in an affluent neighborhood, the compound was surrounded by walls as high as 5.5 meters, topped with barbed wire. Two security gates guarded the only way in. A third-floor terrace was shielded by a 2-meter privacy wall. No phone lines or Internet cables ran to the property. The residents burned their garbage rather than put it out for collection. Intelligence officials believed the million-dollar compound was built five years ago to protect a major terrorist figure. The question was, who?

The CIA asked itself again and again who might be living behind those walls. Each time, they concluded it was almost certainly bin Laden.

President Barack Obama described the operation in broad strokes on Sunday night. Details were provided in interviews with counterterrorism and intelligence authorities, senior administration officials and other US officials. All spoke on the condition of anonymity to discuss the sensitive operation.

By mid-February, intelligence from multiple sources was clear enough that Obama wanted to "pursue an aggressive course of action", a senior administration official said. Over the next two and a half months, Obama led five meetings of the National Security Council focused solely on whether bin Laden was in that compound and, if so, how to get him, the official said.

Normally, the US shares its counterterrorism intelligence widely with trusted allies in Britain, Canada, Australia and elsewhere. And the US normally does not carry out ground operations inside Pakistan without cooperation with Pakistani intelligence. But this mission was too important and too secretive.

On April 29, Obama approved an operation to kill bin Laden. It was a mission that required surgical accuracy, even more precision than could be delivered by the government's sophisticated Predator drones. To execute it, Obama tapped a small contingent of the Navy's elite SEAL Team Six and put them under the command of CIA Director Leon Panetta, whose analysts monitored the compound from afar.

Panetta was directly in charge of the team, a US official said, and his conference room was transformed into a command center.

Details of exactly how the raid unfolded remain murky. But the al-Qaida courier, his brother and one of bin Laden's sons were killed. No Americans were injured. Senior administration officials will only say that bin Laden "resisted". And then the man behind the worst terrorist attack on US soil died from an American bullet to his head.

It was mid-afternoon in Virginia when Panetta and his team received word that bin Laden was dead. Cheers and applause broke out across the conference room.